The invention pertains to machine vision and, more particularly, to methods for testing machine vision inspection systems.
In automated assembly processes, such as those for assembling electronic circuit boards, it is important to determine the exact location of components prior to their placement for final assembly. For example, an assembly system must know the position and angular orientation of an electronic component before placing and soldering its leads to a printed circuit board.
Accuracy in positioning is ever more critical as the size of electronic components decrease and the number of conductive leads increase. Such is the case with packaged integrated circuits, which may have hundreds of fine wire leads that must be aligned with pads on a printed circuit board.
Component inspection is important in other automated assembly processes, as well. For example, in an automotive assembly line, welds and rivets must be verified for safety. Likewise, in a pharmaceutical assembly line, the placement of caps and seals on bottles must be confirmed to minimize the risks of spoilage and tampering.
The art has developed a variety of systems for facilitating automated assembly processes. The most promising of these are "machine vision" systems that analyze video images of parts under assembly to verify their integrity and placement. A machine vision system for surface mounted device inspection, for example, analyzes images from video cameras on robotic assembly equipment to determine and coordinate the precise placement of electronic circuit components.
Notwithstanding the precision of the conveyor belts, robotic arms and other automated assembly equipment, components may wind somewhat out of position or slightly misshapen as they are being readied for assembly. A packaged integrated circuit that is being aligned for soldering to a printed circuit board, for example, may be slightly skewed and have one short lead. A machine vision system must be able to recognize the components and be able to determine their positions in spite of such deviations. Where the deviations are too severe, however, these systems must signal an alert so that the process can be aborted.
Traditional machine vision systems for surface mounted device inspection rely on standardized libraries to facilitate component identification. More advanced systems, such as those sold by the assignee hereof (Cognex Corporation) under the trade names SMD2 and SMD PGP, permit users to define components to be inspected, using a description language (UDL) of the type described in U.S. Pat. No. 5,371,690.
Both conventional and advanced surface mounted device systems permit users to define "characteristics," such as deviations in position and angular orientation, that are acceptable for the assembly process. For example, the systems can be set up to identify and locate a component (e.g., a large-leaded rectangular component) and to signal an alert only if its position deviates by more than 0.5 centimeters from center or its angular orientation deviates by more than 5 degrees from horizontal. The more advanced systems permit users to define additional characteristics, such as variations in lead angles and lengths, body colors or intensities, and edge polarities, that are acceptable for the assembly process.
With the advent of object-oriented programming, many of the machine vision inspection systems use "class" constructs to segregate component definitions and permissible characteristics by component type. The aforementioned SMD PGP product, for example, relies on separate classes to segregate and store data and method members necessary for inspection ball grid arrays, rectilinear devices, etc.
Before machine vision inspections systems can be put to use for surface mounted device or other types of inspections, the systems must be tested to ensure that the definitions of permissible characteristics are consistent with those mandated by the assembly equipment and quality control standards. For example, if a pick-up nozzle cannot adequately grip a component that is rotated more than 7 degrees, the user must be apprised of this so that he or she can modify the definitions accordingly.
According to the prior art, testing is usually accomplished by arranging "by hand" a test component in various positions and running the inspection to determine whether it returns proper results. An alternative is to compile during runtime a database of images and to run the inspection system on each one of those images. A drawback of these prior art procedures is the difficulty in testing all possible variations. Another drawback is the time involved in creating the image database and in performing tests "by hand."
In view of the foregoing, an object of this invention is to provide improved methods for machine vision analysis and, particularly, improved methods for testing machine vision inspection systems.
More particularly, an object of the invention is to provide methods for testing machine vision inspection systems to evaluate their operation under a wide range of operational circumstances.
Yet another object of the invention is to provide methods for testing machine vision surface mounted device inspection systems.
A related object is to provide methods for generating images that can be used in testing machine vision inspection systems.
Yet still another object of the invention is to provide such methods that can execute quickly, and without undue consumption of resources, on a wide range of machine vision analysis equipment.